1. Field of Invention
The present invention relates to a method for removing sulfur dioxide and nitrogen oxides and particulate matter from gas mixtures.
2. Prior Art
Effluent gases containing sulfur dioxide, and/or nitrogen oxides, and/or particulate matter are generated from many sources including coal-burning power plants, iron and steel plants, paper mills, sulfuric, nitric, and other acid production plants, internal combustion engines, and the like. These pollutants may contribute to the formation of photochemical smog and acid rain, and are also harmful to the human body when inhaled. Many processes have been developed for the removal of these pollutants from the effluent gas before they are released to the atmosphere. For example, many coal-burning electric power utilities utilize wet or dry scrubbers for SO.sub.2 removal and cyclones, electrostatic precipitators, or bag filters for the removal of particulate matter. However, the methods are capital intensive and costly to operate. The nitrogen oxides (hereinafter referred to as NO.sub.x) are particularly difficult to remove and in many cases the NO.sub.x production is only minimized by adjusting the operational parameters of the effluent gas producing facility. Some plants burn low sulfur coal to reduce the SO.sub.2 emissions, but this usually results in the formation of high resistivity fly ash particles which are difficult to collect with an electrostatic precipitator. Recently a method has been developed for removal of SO.sub.2 and NO.sub.x by irradiating the effluent gas with a high energy electron beam (-1 MeV) to produce excited chemically active species such as radicals which promote reactions that convert the SO.sub.2 and NO.sub.x into particle or mist form, thereby enabling collection by the previously mentioned conventional particle collection methods. See, e.g., U.S. Pat. Nos. 4,004,995 and 4,435,260. This method is capital intensive since many electron beam accelerators are required, and the induced chemical reactions are limited to the radiation chemistry type. [B.D. Blaustein Chemical Reactions in Electrical Discharges, (Paper) No. 11, Advances in Chemistry Series (80), ACS, Washington, D.C., R. F. Gould, Ed. (1969)].
There are several methods of promoting chemical reactions in gases which are based on forming excited chemically active species in the gas. Many of these methods can be categorized as involving either ionizing radiations or electrical discharges. The ionizing radiation methods include the use of .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet light, X-rays, high energy electron beams, and the like. Electrical discharge chemical processes utilize several types of electrical discharges at low and high gas pressures [Blaustein, Chemical Reactions in Electrical-Discharges, Chapter (Paper No. 36), R. F. Gould, Ed. (1969)]. Low pressure electrical discharges include, e.g., electromagnetic field types such as radiofrequency, microwave, or laser induced discharges; and dc or ac glow discharges [Blaustein, Chemical Reactions in Electrical Discharges, Chapter (Paper) Nos. 22, 24, 29, 35; 21, 27; 3, 12; and also see J. L. Steinfeld, Ed. Laser-Induced Chemical Processes (1981), Plenum Press, New York]. High or atmospheric pressure electrical discharges include, e.g., dc or ac coronas, arc discharges, silent discharges, and streamer coronas.
The high pressure electrical discharges are well suited for promoting chemical reactions in processes at atmospheric pressure. A normal corona discharge is formed when dc or ac high voltage is applied to asymmetrical electrodes in a gas near atmospheric pressure, i.e., a point electrode at negative dc high voltage placed near a grounded plate electrode. The high voltage produces an electric field between the electrodes which is non-uniform and is strongest near the point, resulting in the breakdown of the gas near the point and the production of a corona glow [L. Loeb, Electrical Coronas: Their Basic Physical Mechanism, Univ. of Calif. Press (1965); Meek et al, Eds. Electrical Breakdown of Gases (1978); John Wiley & Sons, Ltd.]. An electrical current made up of ions and electrons flows between the elctrodes across the gap. In the region near the point the strong electric field imparts energy to the electrons. The electrons undergo elastic collisions with gas molecules, but do not lose significant energy because of the large difference in mass of the electrons and ions. Sufficiently energetic electrons can undergo inelastic collisions in which they transfer their energy to the gas molecules by raising the molecules to excited states. The molecules can release this energy by emitting light causing the characteristic glow of the corona discharge. The excited molecules can also involve themselves in chemical reactions or dissociate (forming radicals which are capable of promoting chemical reactions). The most energetic electrons ionize the molecules releasing more free electrons which also gain energy from the field and ionize additional molecules. This is commonly termed a Townsend avalanche and results in an exponential multiplication of the current. When the electrons leave the high field region (as they travel across the gap), they enter the low field region where they do not receive much energy from the field. These low energy electrons attach to molecules forming negative ions which then travel to the grounded plate. Ions gain energy from an electric field but, in contrast to electrons, lose their energy during elastic collision with gas molecules. The similar masses of the ions and molecules results in the transfer of kinetic energy from the ions to the molecules, thereby increasing the temperature of the gas. When ions collide with gas molecules they do not raise the molecules to excited states and, therefore, are incapable of promoting chemical reactions. Consequently, ion current does not contribute to the chemical reaction process, but wastes power heating the gas. Since the electrical current passing between the electrodes only contains electrons near the point, the production of excited molecules and radicals is limited to a small region near the point. Most of the current between the electrodes is ionic current, therefore the formation of excited molecules (active species) and radicals is power inefficient. Since the formation of active species only occurs in a small fraction of the interelectrode volume the formation rate is low and the volume efficiency is poor.
Recently experiments were conducted in Japan in which dc and ac corona discharges were used to remove NO.sub.x from a gas stream [Tamaki et al, The Chemical Society of Japan, Vol. 11, p. 1582 (1979)]. The power efficiency and removal rate, however, were found to be low. The use of a corona discharge for SO.sub.2 and NO.sub.x removal has also been investigated. [K. Ootsuka, Electrical Method of Integrated Pollution Control for Combustion Gases, Ph.D. Dissertation, Dept. of Electrical Eng., Univ. of Tokyo, (1984)].
The arc discharges can also be used to promote chemical reactions [Blaustein, Chemical Reactions in Electrical Discharges, Chapter (Paper) No. 33, American Chemical Society Publications]. An arc discharge is very different, however, from the corona discharge. The arc consists of a narrow filament of high current connecting the two low voltage electrodes. (A corona discharge is low current and high voltage). The arc filament is heated to a very high temperature by the high current (e.g., an arc welder). Arc discharge chemistry is very power inefficient because of the high ion current and the power expended heating the gas. The treatment volume is also small due to the restricted volume of the single arc filament.
The silent corona discharge occurs when ac HV is applied across parallel or concentric electrodes separated by a dielectric layer (e.g., glass) and a small air gap (-1 mm). The resulting electric field is uniform across the gap, in contrast with the nonuniform electric field used to produce normal dc or ac coronas. The dielectric layer prevents sparkover from occurring and allows a diffuse uniform discharge to occur in the gap. The silent discharge can be used to promote gas phase chemical reactions. [Blaustein, Chemical Reactions in Electrical Discharges, Chapter (Paper) Nos. 17, 25, 26 and 28. American Chemical Society Publications].
One of the earliest applications of the silent discharge was the production of ozone [Rice et al, Handbook of Ozone Technology and Application, pp. 1-84 (1982); also U.S. Pat. Nos. 387,286 (1888) and 587,770 (1897)]. When dry air is passed through a silent discharge some of the O.sub.2 molecules dissociate to O.sup.-. Many of the O.sup.- atoms combine with O.sub.2 molecules to form ozone (O.sub.3). Improved silent discharge processes are among the best methods of producing ozone today. These processes use ac or high frequency ac (1-10 kHz) HV to produce the silent discharge; however, these processes are very power inefficient due to the ion current of discharge wasting power heating the gas. The waste heat is usually removed by liquid cooling of the electrodes [U.S. Pat. No. 3,766,051 (1973)]. The power efficiency of the process can be improved by limiting the ion current in the silent discharge by applying the proper voltage waveforms. [U.S. Pat. No. 4,016,060 (1977)].
When high voltage dc is applied across two plate electrodes in air very little current flows between the electrodes. As the voltage is increased a spark suddenly occurs between the plates (breakdown) which effectively "shorts" the plates, and then a large current flows which trips the power supply circuit breaker. High speed photography studies have shown that immediately before breakdown a streamer travels between the plates [L. B. Loeb, Electrical Coronas: Their Basic Physical Mechanisms, pp. 143, 167 (1965), Univ. of Calif. Press; Meek et al, Eds., Electrical Breakdown of Gases, p. 439 (1978), John Wiley & Sons, Ltd.]. The streamer is very short lived because it triggers the spark breakdown.
A stable streamer discharge can be formed by applying very short duration HV pulses (200 ns) to asymetrical electrodes. During the 200 ns that the HV is on, the streamers travel between the electrodes. There is not enough time for spark breakdown to occur before the voltage is off (between pulses) and the streamers disappear.
The pulses can be applied many times each second which results in a streamer corona discharge (a brush-like discharge consisting of many filamentary streamers) which can be used to promote chemical reactions. Most of the current in the streamer discharge is due to electrons because ions are about 500 times less mobile than electrons. This results in a high power efficiency since electron current can promote chemical reactions whereas an ion current cannot. The streamers travel across the entire interelectrode volume which results in a large formation rate of active species. The streamer corona discharge differs from the normal corona or silent corona discharges in the following respects.
(1) The active species producing region consists of brush-like streamers which travel across the entire electrode gap instead of consisting of a small glow region near the point elecrode or a uniform glow near the dielectric layer. Therefore, the active high electric field region at the streamer tip also travels across the entire gap. PA1 (2) The streamer corona discharge has a large electron contribution to the current whereas the corona and silent corona discharges have a large ion current. PA1 (3) The streamers are formed by very short pulses (200 ns) of HV whereas the corona and silent discharge are normally formed by constant or periodically varying HV.
The normal corona and silent corona discharges can be operated in a pulsed mode (usually pulses longer than 1 .mu.s are used), but this should be distinguished from the streamer corona discharge because the actual physical mechanisms of the discharges are different.
A major application of normal corona discharges is electrostatic precipitation [H. White, Industrial Electrostatic Precipitation, Addison-Wesley, Pergamon Press, Oxford, 1963; also see S. Oglesby et al, Electrostatic Precipitation, Pollution Engineering and Technology Series, Ed. Young et al, Marcel-Dekker, N.Y. (1978)]. Electrostatic precipitators (hereinafter referred to as ESP) normally employ negative polarity dc corona discharges in wire-plate geometries for removing particulate matter from gas streams. The particles are charged and driven to the plate electrode where they form a layer which is rapped (vibrated) and falls into a hopper. Negative polarity is used because it usually results in better particle collection efficiencies than does positive polarity.
Pulsed negative coronas have also been employed in electrostatic precipitators [H. White, supra; U.S. Pat. Nos. 2,000,017; 2,509,548]. Operating the corona discharge in the pulsed mode enables the application of higher voltage without sparkover, results in a more uniform corona discharge along the wire surface, and lowers the ion current density in the interelectrode volume [H. Milde, IEEE Transactions on Electrical Insulation, Vol. E1-17, No. 2, p. 179, April 1982]. The lower current density helps prevent detrimental back corona from occurring by decreasing the voltage drop across the particle layer which is normally present on the collecting electrode. Back corona occurs when the voltage drop across this layer becomes large enough for breakdown of the layer, and releases particles and oppositely charged ions into the gas stream.
Pulsed corona discharges are different from streamer corona discharges. The streamer corona consists of many filamentary streamers which extend across the interelectrode volume, whereas the pulsed corona discharge is a uniform glow in the vicinity of one electrode.
Streamer corona discharges (negative polarity) have been employed in a Boxer Charger between closely spaced (20 mm) helical electrodes to generate a plasma from which negative ions are extracted by a high voltage AC potential. The extracted ions are then used to precharge particles before they enter an ESP [S. Masuda, A. Mizuno, et al, Conf. Rec. of IEEE/IAS, Annual Meeting, Philadelphia, Pa., pg. 1066 (1981)]. Masuda et al have also generated a plasma in the immediate volume enclosing the wire electrodes in an ESP using negative streamer coronas. A very uniform negative ion current is extracted from the plasma region into the interelectrode volume using negative polarity dc high voltage [Masuda et al, Conf. Rec. of IEEE/IAS Annual Meeting, Mexico City, p. 966 (October, 1983)].
Streamer corona discharges have also recently been employed in transmission line ozonizers to produce O.sub.3 from dry oxygen. [S. Masuda et al, In: Conf. Rec. of IEEE/IAS, Annual Meeting, Chicago, Ill., p. 978 (Oct. 1984)]. Streamer corona discharges have also been used to promote chemical reactions which remove SO.sub.2 from an airstream [A. Mizuno, J. S. Clements and R. H. Davis, In: Conf. Rec. of IEEE/IAS, Annual Meeting, Chicago, Ill., p. 1015 (October, 1984)]. In the system described therein a point to plane electrode geometry was used to promote the streamer corona discharge in air. High voltage pulses (45 kV peak, 200 ns duration) were applied to the point electrode with a repetition frequency of 60 times per second. The streamer corona discharge removed significant amounts of the SO.sub.2 from an airstream containing 1500 ppm SO.sub.2 and water vapor. The SO.sub.2 removal of the system was compared with that of a high energy electron beam and a normal dc corona discharge. The streamer method was more power efficient than the other methods based on delivered power. Moreover, the streamer method formed a powder without the addition of additives (e.g., NH.sub.3) which were required in other processes.
To date, the application of streamer corona discharge chemistry has been limited to the production of ozone in transmission line ozonizers, or the use of point or rod to plane geometries for the removal of sulfur dioxide from humid air.
It is an object of the present invention to provide an improved streamer corona discharge based method for removing sulfur dioxide and/or nitrogen oxides from mixtures thereof with other gases.
It is a further object of the invention to provide a streamer corona discharge based method for removing sulfur dioxide and/or nitrogen oxides, and/or particulate matter from mixtures thereof with other gases.